1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for producing the same. In particular, the present invention relates to a ferromagnetic metal thin film magnetic recording medium having a protective layer and a lubricant layer successively formed on a ferromagnetic metal thin film, which medium is used audio and video equipment, a computer, and so on, and achieves both electromagnetic conversion characteristics and practical reliability at high levels.
2. Description of the Related Art
In these years, a magnetic recording equipment is required to have a large capacity, achieve high speed reading and writing, high picture and sound quality, and have a small size and a light weight. To satisfy such requirements, it is inevitable for a magnetic recording medium to achieve a high density recording. To this end, instead of a conventional powder coating type magnetic recording medium having a magnetic layer which comprises a binder and magnetic powder dispersed in said binder, a ferromagnetic metal thin film magnetic recording medium has been actively developed and practically used, since it has a larger residual magnetic flux density (Br) and coercive force (Hc) and a thinner magnetic layer, and is more suitable for ultra-smoothing of a magnetic layer surface than the powder coating type one.
However, since a magnetic layer of the ferromagnetic metal thin film magnetic recording medium has low hardness and is easily deformed plastically, when it is directly contacted to a magnetic head of a VCR (video cassette recorder) which rotates at a high speed, it is immediately worn or flawed and adhered to a surface of the head. As a result, the durability is deteriorated, for example, a writing and reading output decreases greatly after repeated running and a still-frame life is considerably shortened. Though a surface of the magnetic layer is protected by the formation of an oxide film, corrosion resistance in a high humidity circumstance is insufficient.
To increase the lubricity, wear resistance and corrosion resistance of the ferromagnetic metal thin film magnetic recording medium, it is proposed to use a base film having minute protrusions on its surface or to form a protective layer or a layer of a fluorine-containing lubricant which provides both the lubricity and water repellency. In particular, the protective film should have a small thickness to decrease a spacing loss between the magnetic layer and the magnetic head. Then, many proposals have been made on the formation of a diamond-like carbon film which is hard and hardly worn on the magnetic layer (see Japanese Patent KOKAI Publication Nos. 210518/1986 and 98824/1988).
Japanese Patent KOKAI Publication Nos. 245417/1989 and 158909/1990 disclose a magnetic recording medium having a diamond-like carbon film on a magnetic layer and a lubricant layer of a fluorine-containing aliphatic acid on the diamond-like carbon film.
However, it is very difficult to provide a magnetic recording medium which satisfies both the running stability and the durability by the above proposed methods, and many problems arise.
For example, when only the fluorine-containing lubricant layer is formed on the magnetic layer, a shear force can be decreased, but the surface hardness of the medium is low and the medium is easily abraded, so that the running stability is deteriorated and the still-frame life is shortened.
When only a hard protective layer such as the diamond-like carbon film is formed on the magnetic layer, the protective layer itself suffers from brittle failure due to the direct contact to the magnetic head of the VCR which rotates at a high speed. As the result, the still-frame life is greatly shortened and the stability of reading output cannot be maintained when the medium is repeatedly run.
When the very hard diamond-like carbon film and the lubricant layer are successively formed on the magnetic layer, since a surface state of the diamond-like carbon film is very inactive chemically, its adhesion to the lubricant layer is insufficient, so that a material containing the lubricant component is seized on the magnetic head surface during writing and reading, whereby the output is decreased or the head gap is clogged for a long time.
When the hard carbon film is formed on the magnetic layer, since the adhesion between the magnetic layer and the hard carbon film is insufficient, the hard carbon film is partly peeled off during reading in a still mode or after long time storage in a high temperature high humidity atmosphere, and then the durability and weatherability are severely deteriorated.
Recently, to improve the adhesion between the protective layer and the lubricant layer, it is proposed (1) to form, on the ferromagnetic metal thin film, a hard carbon protective film, a nitrogen-containing plasma polymerized film and a lubricant layer comprising a fluorine-containing carboxylic acid (see Japanese Patent KOKAI Publication No. 126417/1990); (2) to form, on a ferromagnetic metal thin film, a protective layer comprising an organic polymer compound which has at least carbon atoms and nitrogen atoms with a concentration of the nitrogen atoms in a surface layer being at least 40 atomic % of the carbon atom, and a lubricant layer on the protective layer (see Japanese Patent KOKAI Publication No. 58416/1987); (3) to form a hard carbon thin film containing B, Ti or Si on a ferromagnetic metal thin film, and then provide a lubricant layer having a reactive group successively by a vacuum deposition method (see Japanese Patent KOKAI Publication No. 184722/1989); or (4) to form a protective layer comprising graphite-like carbon which contains at least one element selected from the group consisting of Mn, Mo, Nb, Ta, Ti, V and W on a ferromagnetic metal thin film and successively form a lubricant layer comprising an organic compound having a mercapto group (see Japanese Patent KOKAI Publication No. 177312/1988).
Further, Japanese Patent KOKAI Publication No. 126418/1990 discloses a ferromagnetic metal thin film magnetic recording medium in which a hard carbon film is formed on the ferromagnetic metal thin film, the surface of the hard carbon film is treated with ammonia gas by glow discharge and then a lubricant layer comprising a fluorine-containing carboxylic acid is formed thereon.
Japanese Patent KOKAI Publication No. 68834/1994 discloses a ferromagnetic metal thin film magnetic recording medium having a protective layer (a plasma polymerized layer), wherein an oxygen to carbon (O/C) ratio is 0.30 or less in the surface layer of the protective layer contacting the magnetic metal film, to increase the adhesion between the magnetic layer and the protective layer.
However, by the above recently proposed techniques, it is still difficult to provide a magnetic recording medium which is excellent both in the electromagnetic conversion characteristics and the durability. Then, still many problems should be solved.
For example, when the nitrogen-containing plasma polymerized film is formed between the hard carbon protective film and the lubricant layer as proposed in Japanese Patent KOKAI Publication No. 126417/1990, the plasma polymerized film is less hard and easily worn, so that the running stability and the durability are not sufficiently satisfied. If a thickness of the plasma polymerized film is increased to improve the wear resistance, the spacing loss between the magnetic layer and the magnetic head is increased and then the electromagnetic conversion characteristics is worsened.
When the nitrogen atom concentration in the protective layer is increased as disclosed in Japanese Patent KOKAI Publication No. 58416/1988 or when the lubricant layer is formed on the protective layer containing B, Ti, Si or the metal as disclosed in Japanese Patent KOKAI Publication Nos. 184722/1989 and 17731 2/1987, while the adhesion between the protective layer and the lubricant layer is improved, the hardness of the protective layer is decreased so that the durability such as the still-frame life is deteriorated.
When the lubricant layer is formed after treating the surface of the hard carbon protective film with the ammonia gas by glow discharge as disclosed in Japanese Patent KOKAI Publication No. 126418/1990, since the surface of the hard carbon protective is severely damaged by the bombardment of charged particles generated from the non-polymerizable ammonia gas, the durability and the weatherability are heavily deteriorated.
Since none of the above proposed techniques considers the adhesion between the ferromagnetic metal thin film and the protective layer, the weatherability of the medium is unexpectedly greatly worsened.
When the ratio of oxygen atoms to carbon atoms in the interface (a layer in which the ferromagnetic metal elements exceed 5 atomic % when the atomic composition is analyzed in the depth direction of the plasma polymerized film by the X-ray photoelectron spectroscopy or the Auger electron spectroscopy) of the plasma polymerized film contacting the magnetic layer surface is simply specified as disclosed in Japanese Patent KOKAI Publication No. 68834/1994, contaminants or low molecular weight materials which adhere to the magnetic layer surface are not removed, that is, the magnetic layer surface is not cleaned, so that the adhesion between the magnetic layer and the protective layer is not sufficiently improved and then the durability and the weatherability are not improved considerably.